Variable duty cycle arc lamp modulator

ABSTRACT

The disclosed variable arc lamp modulator comprises a bistable switch connected in series with an arc lamp and a power supply. The switch is triggered ON (closed) by a series of pulses of constant frequency from a clock pulse generator, and the switch is triggered OFF (open) by a series of pulses from another pulse generator. The time interval between the ON and OFF pulses is varied in accordance with the impedance of the arc lamp as detected by a current integrator and a level sensor.

United States Patent Laakmann et al.

[4 1 Sept. 30, 1975 [75] Inventors: Peter Laakmann, Los Angeles,

C alif.; Charles U. Boutin, Tucson, Ariz [73] Assignce: Hughes AircraftCompany, Culver City, Calif.

[22] Filed: Dec. 22, 1966 [211 App]. No; 605,139

[52] U.S. Cl 315/209; 315/246 [5]] Int. Cl. HOlj 29/00 [58] Field ofSearch,..-... 315/209, 362, 246; 307/96, 307/97, 98, 99

[56] References Cited UNITED STATES PATENTS 3,265,930 8/1966 Powell3l5/209 VARIABLE DUTY CYCLE ARC LAMP MODULATOR 3,278,800 l0/1966Snell... ,3l5/209 Primary ExaminerMaynard R. Wilbur AssistanrE.\'aminerJ. M. Potenza Attornqv, Agenflor Firm-James K. Haskell; PaulM. Coble 5 7 ABSTRACT The disclosed variable are lamp modulatorcomprises a bistable switch connected in series with an arc lamp and apower supply, The switch is triggered ON (closed) by a series of pulsesof constant frequency from a clock pulse generator, and the switch istriggered OFF (open) by a series of pulses from another pulsegenerator.The time interval between the ON and OFF pulses is varied in accordancewith the impedance of the arc lamp as detected by a current integratorand a level sensor.

4 Claims, 2 Drawing Figures The invention herein described was made inthe course of or under a contract or subcontract thereunder, (or grant)with the Department of the Army.

This invention relates generally to are lamps and more particularly to amethod and apparatus for electronically modulating arc lamps.

Arc lamps, such as mercury vapor or xenon arc lamps used as opticalsignal sources, require that the current supplied thereto be controlled(this control is often referred to as modulating or keying the arclamp). This modulation serves the purpose of increasing the signal tonoise ratio at the optical sensor by rejecting all signals that are notmodulated (keyed) at the desired rate. The signal in these systems isusually recovered through a narrow band filter at the keying rate.

This keying, however, can cause severe instabilities in the arcdischarge mechanism that can cause the arc to extinguish. This occursparticularly if the keying frequency is above about 1 Kl-l Theinstabilities are caused by the exitation of acoustical plasmaresonances that are exited by the keying frequency. Additionalinstabilitics can be caused by high frequency mechanical vibration ormechanical shock.

These instabilities are in addition to the instabilities found even inunmodulated arcs such as temporary shifts in the attachment point of theare on the cathode due to, for example, thermal convection.Instabilities due to all causes are characterized by an increase in arcimpedance, and all of them occur at random times and may stretch overtime intervals from seconds to fractions of milliseconds. When theseinstabilities occur, it is imperative to maintain the total powersupplied to the arc lamp, or better to increase the power level, as thishas the tendency to increase the electrode and plasma temperaturestemporarily and will thereafter better maintain the emission process.

One of the present methods of powering arc lamps is to use as much of aconstant current source as is possible. A constant current source hasthe capability to adjust the power flow to the arc lamp immediately.Since, in this case the ON current is constant, the power isproportional to the voltage drop. The voltage drop is always higherduring periods of instability. A lamp operated in this mode will neverextinguish during periods of increased arc impedance.

Another method is to modulate or key the arc lamp (intermittently applypower thereto). However, if the duty cycle is controlled in order tomaintain and increase power to the arc lamp over some range of arcimpedance, such controlling action has to be very fast. In keyedapplications instability has been observed to occur on a cycle to cyclebasis, in extreme cases. It becomes apparent that any such controlshould have as little delay as possible. The above complications have inthe past required a DC supply of relatively high voltage and a largeseries resistance, all in series with a modulating electronic switch,for example, a transistor. An alternate approach would be the use of aconstant current regulator.

Whether a high supply voltage is used or an electronic regulator,reliable arc lamp operation was in the past only possible by dissipatingconsiderable power in the regulator or series resistance. Usually thepower loss has to exceed the power delivered to the arc by a factor oftwo to five in order to achieve stable and reliable operation. In manyinstances such power loss is intolerable and it is always undesirable.

It is therefore a primary object, of the present invention to provide amethod and apparatus for electronically modulating arc lamps.

It is another object of the present invention to control the powersupplied to an arc lamp by varying the ON time of a keying circuit as afunction of the arc impedance.

It is a further object of the present invention to provide a method andapparatus for controlling the duty cycle and power flow as a function ofarc impedance with zero delay and with increased efficiency Theseobjects are accomplished according to the present invention as follows.An arc lamp is provided with a power supply and a bistable switch inseries with the arc lamp. The electronic modulating system of thisinvention includes a clock pulse generator operating at the modulatingfrequency to deliver a series of narrow pulses at equal time intervalsto trigger the bistable switch into the ON (closed) state. A separatepulse generator is employed to turn the bistable switch OFF after apreset value of the time integral of the current which has passedthrough the arc lamp has been reached. Consequently, when the arcimpedance is low, a high current is flowing and the bistable switch willbe ON (closed) only for a short time. On the other hand, if the arcimpedance is high only a small current will flow but for a longerportion of the cycle. If the are impedance changes during the ON cycleitself, the change will be immediately reflected in the integration andthe bistable switch will only turn OFF after the preset value of thetime integral of current has been reached.

The present invention thus provides a method and apparatus forcontrolling the duty cycle by varying the ON time of the keying circuitas a function of arc impedance with zero delay. Compared to the constantcurrent method it is also much more efficient in delivering power from asupply to the arc lamp.

These and other features of the present invention will be more fullyunderstood by reference to the following detailed description when readin conjunction with the accompanying drawings in which like referencecharacters refer to like elements, and in which:

FIG. 1 is a block diagram which illustrates a preferred embodiment ofthe present invention; and

FIG. 2 is a schematic circuit diagram of the embodiment shown in FIG. 1.

Referring now to the drawings, FIG. 1 shows an arc lamp 2, the operationof which is to be controlled by the present invention. The are lamp 2may consist of, for example, a mercury vapor arc lamp. The circuit to bedescribed was originally designed for an arc lamp to be used as anoptical signal source on a one-shot mission. This invention, however, isapplicable to any restartable arc modulator. A power supply 4 providespower for the circuit. The keying system consists of a clock pulsegenerator 6, operating at the modulating frequency, which deliversnarrow pulses 8 to trigger a bistable semiconductor switch 10 into theON (closed) state and a separate pulse generator 12 to turn the bistableswitch 10 OFF (open) after the preset value of the time integral ofcurrent has been reached.

In order to measure the time integral of current a current integrator 14is provided in series with the arc lamp 2. The current integrator 14feeds a signal, corresponding to the time integral of current, to alevel sensor 16. When the preset value of the time integral is reachedat the level sensor 16, the level sensor 16 feeds a signal to theturn-off pulse generator 12. This signal triggers the pulse generator 12into delivering a pulse to the switch to turn it OFF. Consequently, whenthe arc impedance is low, a high current is flowing and the switch 10will be closed for only a short time. On the other hand, if the arcimpedance is high only a small current will flow but for a longerportion of the cycle.

If the arc impedance changes during the ON cycle it-' power dissipatedin the arc would be constant for a constant voltage power source. If thepower source has internal resistance, the power dissipated in the lampwould increase with arc impedance. This is a more de sirable situationas was pointed out earlier. The presence of power source resistance (oradditional series resistance), of course, does cause losses. Theselosses, however, are much smaller than the losses caused by operationwith a high source voltage and series resistance for the same powerregulation. In a practical case where the power source might be abattery, much resistance is usually present anyway.

. If efficiencies approaching 100 percent from a constant voltage powersource are desired, the rising power versus rising arc resistancecharacteristics can be obtained easily by a nonlinear integration.Suchnonlinear integration can be obtained by weighting? high currentsdifferently from low currents. For example, in a capacitive integrator ashunt resistor paralleled to the capacitor will do this effectively.

If a detector tuned to the fundamental of the modulating frequency isused, it is good design practice to limit the expected range of dutycycle to below 50 percent by the proper choice of supply voltage,average current and are impedance. The fundamental component falls offrapidly if the duty cycle exceeds 50 percent. Duty cycles less than 50percent cause a slight increase in fundamental component for suchdetectors as silicon or lead sulfide, depending of course on the powerversus peak arc current characteristic of the circurt.

Referring now to the circuit diagram of FIG. 2, the bistablesemiconductor switch 10 shown in FIG. 1 comprises a power transistor V asilicon gate controlled switch V anda silicon transistor V in connectionwith the latching loop which comprises a resistor R and diodes D and DThe saturated transistor V removes the drive from the transistor V andcharges the capacitor C through the resistor R to turn the gatecontrolled switch V ON. This in turn will turn ON the transistor V, bykeying the base to collector path to be shorted. The latch ing loopwhich comprises a resistor R and the diodes D and D will keep thetransistor V cut off.

The transistor V is saturated periodically at the clock pulse rate by astandard unijunction relaxation oscillator which comprises a unijunctiontransistor V coupled in a relaxation oscillator circuit including aresistor R and a capacitor C The transistor V is turned ONby theintegrating unijunction transistor V This will cause a negative turn ofpulse to appear at the gate of the switch V to turn off the arc lampafter the preset integral of current is passed therethrough.

The integration is performed in the capacitor C The charging current,which is proportional to the are current, is derived from the invertingtransistor V The collector current in the transistor V is proportionalto the emitter current, which in turn is proportional to the voltagedrop across the resistor R Saturation at the clock pulse rate of thetransistor V resets the integrator at the start of each ON cycle. Thearc lamp 2 used in this embodiment is of the type which is started bymeans of a fusible wire link between the electrodes. During the fusingperiod the integrator is inhibited by the capacitor C and the diode DThe above described embodiment of the present invention was successfullyoperated using the following values for the components of the circuit.

V, 2N2359 R ohms V 2N3262 R 75 ohms V 2N2222 R, 330 ohms V 2N492A R 18Kohms V 2N492A R 5 l0 ohms V 2N398B R ohms V 2N2222 R I50 ohms V TlCll R75 ohms D IN36OU' C In F o, IN36OO C .22 F' D IN360O C .022 F 1),,IN3600 C, 6.8 1. F R ,1 ohm C .(llu F R 25 ohms C lSu F R 560 ohms L, H

Since the present invention relates to are lamps, the

following definition thereof will be helpful. Arc lamps are two or threeelectrodedevices operating atgas pressures above one atmosphere, whoseemission proof flash lamps is dueto higher-currents. Stability in flashlamps is no problem.

What is claimed is: 1. Apparatus. for controlling the supply of powerfrom a power source to an arc lamp comprising:

an arc lamp; a power supply connected to said are lamp; a'bistableswitch connectedin series with the lamp and the power supply having anON state in which the power supply is electrically connected to the arclamp and an OFF state in which the power supply is not electricallyconnected to the arc lamp; first means to supply first electricalpulsesto the switch at regular intervalsto trigger the switch into theONstate; and second means to supply second electrical pulses to theswitch to trigger the switch into the OFF state, said second meanscomprising a current integrator for sensing current flow through the arclamp and pulse generating means for generating the second 2. Theapparatus according to claim 2 in which the closing said switch atpredetermined time intervals; pulse generating means comprises:integrating, with respect to time, the amount of curlevel sensormeansconnected to the current integrarent which flows through said arclamp each time tor for receiving from the current integrator a sigsaidswitch closes; and nal corresponding to the time integral of current 5opening said switch when the time integral of current which has passedthrough the arc lamp, and for which passes through said are lamp reachesa pregenerating an electrical pulse when the signal from determinedvalue. the current integrator reaches a predetermined 4. The method ofcontrolling the supply of power value; and from a power supply to an arclamp comprising: a turn-off pulse generator connected between the 10initiating a flow of current from said power supply to level sensor andthe switch for delivering a pulse to said are lamp at intermittent timeintervals; the switch when the generator receives a stimulus integratingthe amount of current which passes from the level sensor to trigger theswitch into the through said arc lamp each time said flow of cur- OFFstate. rent is initiated; and 3. The method of controlling the supply ofpower interrupting said flow of current when the time intefrom a powersupply to an arc lamp comprising: gral of current reaches apredetermined value.

providing a switch in series with said arc lamp;

1. Apparatus for controlling the supply of power from a power source toan arc lamp comprising: an arc lamp; a power supply connected to saidarc lamp; a bistable switch connected in series with the lamp and thepower supply having an ON state in which the power supply iselectrically connected to the arc lamp and an OFF state in which thepower supply is not electrically connected to the arc lamp; first meansto supply first electrical pulses to the switch at regular intervals totrigger the switch into the ON state; and second means to supply secondelectrical pulses to the switch to trigger the switch into the OFFstate, said second means comprising a current integrator for sensingcurrent flow through the arc lamp and pulse generating means forgenerating the second pulses when a predetermined value of the timeintegral of the sensed current is provided by the integrator.
 2. Theapparatus according to claim 2 in which the pulse generating meanscomprises: level sensor means connected to the current integrator forreceiving from the current integrator a signal corresponding to the timeintegral of current which has passed through the arc lamp, and forgenerating an electrical pulse when the signal from the currentintegrator reaches a predetermined value; and a turn-off pulse generatorconnected between the level sensor and the switch for delivering a pulseto the switch when the generator receives a stimulus from the levelsensor to trigger the switch into the OFF state.
 3. The method ofcontrolling the supply of power from a power supply to an arc lampcomprising: providing a switch in series with said arc lamp; closingsaid switch at pRedetermined time intervals; integrating, with respectto time, the amount of current which flows through said arc lamp eachtime said switch closes; and opening said switch when the time integralof current which passes through said arc lamp reaches a predeterminedvalue.
 4. The method of controlling the supply of power from a powersupply to an arc lamp comprising: initiating a flow of current from saidpower supply to said arc lamp at intermittent time intervals;integrating the amount of current which passes through said arc lampeach time said flow of current is initiated; and interrupting said flowof current when the time integral of current reaches a predeterminedvalue.